Field of the Disclosure
The present disclosure, for example, relates to wireless communication systems, and more particularly to techniques for accessing a cell using an unlicensed radio frequency spectrum band.
Description of Related Art
Wireless communication systems are widely deployed to provide various types of communication content such as voice, video, packet data, messaging, broadcast, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., time, frequency, and power). Examples of such multiple-access systems include code-division multiple access (CDMA) systems, time-division multiple access (TDMA) systems, frequency-division multiple access (FDMA) systems, and orthogonal frequency-division multiple access (OFDMA) systems.
By way of example, a wireless multiple-access communication system may include a number of base stations, each simultaneously supporting communication for multiple communication devices, otherwise known as user equipment (UEs). A base station may communicate with UEs on downlink channels (e.g., for transmissions from a base station to a UE) and uplink channels (e.g., for transmissions from a UE to a base station).
Some modes of communication may enable communications with a UE over an unlicensed radio frequency spectrum band, or over different radio frequency spectrum bands (e.g., a licensed radio frequency spectrum band or an unlicensed radio frequency spectrum band) of a cellular network. With increasing data traffic in cellular networks that use a licensed radio frequency spectrum band, offloading of at least some data traffic to an unlicensed radio frequency spectrum band may provide a cellular operator with opportunities for enhanced data transmission capacity. An unlicensed radio frequency spectrum band can also provide wireless access for a venue, such as a stadium or hotel, that may not have access to a licensed radio frequency spectrum band.
Prior to gaining access to, and communicating over, an unlicensed radio frequency spectrum band, a base station or UE may perform a listen before talk (LBT) procedure to contend for access to the unlicensed radio frequency spectrum band. An LBT procedure may include performing a clear channel assessment (CCA) procedure to determine whether a channel of the unlicensed radio frequency spectrum band is available. When it is determined that the channel of the unlicensed radio frequency spectrum band is unavailable, a CCA procedure may be performed for the channel again at a later time.
Under some conditions (e.g., when a cell operating in a licensed radio frequency spectrum band is not available), a UE may access a cell operating in an unlicensed radio frequency spectrum band as the UE's primary cell. One way to access a cell operating in an unlicensed radio frequency spectrum band is to use a random access procedure and radio resource control (RRC) connection setup procedure similarly as used for Long Term Evolution (LTE) communications or LTE-Advanced (LTE-A) communications. However, when operating using an unlicensed radio frequency spectrum band, such a random access procedure may require the performance of one or more CCA procedures by both a base station and a UE to win contention for access to the unlicensed radio frequency spectrum band. Due to the uncertainty of contention for access to the unlicensed radio frequency spectrum band, access procedures requiring fewer contentions for channel access may be desirable.